1. Field of the Invention
This invention concerns a drive mechanism for stepping motors in printers, and more particularly to a drive mechanism for a stepping motor that moves a carriage shaft to which a print head is attached up and down relative to a platen and that adjusts the gap (head gap) between the print head and the paper.
2. Description of the Related Art
Many printers have been proposed (as in the Japanese patent application disclosed H7-87217 [1995], for example) which are configured so that the printer carriage is moved toward or away from the platen by a stepping motor, so that the head gap is variable, and can be adjusted, according to the thickness of the paper, head gap being the gap between the print head carried on the carriage and the paper positioned on the platen.
The printer proposed in the patent application H7-87217 is a so-called flat head printer comprising a platen positioned horizontally, the upper surface of which is flat, and a print head positioned above the platen so that it can move freely back and forth along the platen. This printer is configured such that the head gap is variably adjusted according to the thickness of the paper by using a stepping motor to move the carriage up and down relative to the printer, and causing the print head carried on the carriage to move closer to or farther away from the platen.
The head gap is made so that it can handle a paper thickness, for example, of from 0.05 mm to 2 mm. In order to maintain good print quality, the distance between the paper and the head is said to be to a precision of .+-.30 .mu.m, with the amount of head gap variation being 10 .mu.m for each step of the stepping motor. A standard position for the print head relative to the platen is established. During manufacture, the printer is assembled so that the standard position is within the desired tolerance of .+-.30 .mu.m.
The stepping motor itself has no position detecting means, however, so that, in order to implement precise positioning, a position detecting means has to be provided externally.
Conventionally, when a stepping motor is used to move a carriage and thus to determine the standard position of a print head relative to the platen, position detecting means are provided such that part of the carriage turns a mechanical switch or optical switch fixed to the frame on and off.